Articles | Volume 17, issue 3
Research article
07 Feb 2017
Research article |  | 07 Feb 2017

Differences in BVOC oxidation and SOA formation above and below the forest canopy

Benjamin C. Schulze, Henry W. Wallace, James H. Flynn, Barry L. Lefer, Matt H. Erickson, B. Tom Jobson, Sebastien Dusanter, Stephen M. Griffith, Robert F. Hansen, Philip S. Stevens, Timothy VanReken, and Robert J. Griffin

Abstract. Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (< 0.5 pptv); however, daytime (08:00–20:00 LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2.

Short summary
The atmospheric chemistry associated with mixing of anthropogenic and natural species was simulated to understand how shade provided by a forest canopy impacts reactions, product distribution, and subsequent phase distribution of the products. This is important to understand, as forested areas downwind of urban areas will be impacted by this phenomenon. It was found that fast transport from below the canopy led to increases in secondary organic aerosol from nitrate radicals above the canopy.
Final-revised paper